Pneumatic rubber tires are conventionally prepared with a tread composed of elastomer(s) reinforced with carbon black and sometimes with silica or a combination of carbon black and silica such as for example precipitated silica.
This invention is more specifically directed to truck tires and other relatively large tires including relatively heavy duty light truck tires, which are normally expected to be capable of supporting and carrying relatively large vehicular loads and, thus, tend to generate a greater internal heat build-up and a resulting higher internal operating temperature than ordinary passenger vehicle tires.
The truck tire treads are typically of a cap/base configuration, with the outer tread cap rubber layer intended to be ground-contacting, namely containing a running surface for the tread, which usually contains a lug and groove configuration. The underlying, internal, rubber layer, which is usually a tread base rubber layer, underlies and supports the tread cap rubber layer and is generally positioned between the tread cap and the tire carcass to provide a transition of rubber properties. Generally, the tread cap and tread base rubber layers are co-extruded together as an uncured component of the tire construction and cured, or vulcanized together with the overall tire construction in a suitable mold under conditions of elevated temperature and pressure. Such tread and tire configuration is well known to those having skill in such art.
For such relatively heavy duty tires, internal heat build-up of its tread, as evidenced by tire tread running temperature build-up, or increase, is normally not desired, although a portion of the heat buildup, or temperature increase, is relatively unavoidable during a running of the tire under load.
A challenge is presented of providing such tire with a tread which promotes reduced internal heat build-up during service.
For such challenge, a tire tread rubber composition is envisioned for evaluation which comprised of a cis 1,4-polyisoprene rubber based rubber composition which contains a high surface area precipitated silica with minimal carbon black reinforcement, 3,4 polyisoprene, hydrocarbon resin and sulfur cure package comprised of sulfur and sulfur cure accelerators.
A purpose of utilizing a high surface area precipitated silica for evaluation for reinforcement for the tread cap elastomer in a tread of cap/base construction, with tread cap rubber of natural rubber/synthetic rubber composition, is to provide a tire tread construction which can run about as cool as and with acceptable treadwear as compared to a natural rubber tread reinforced with carbon black. It is contemplated that a truck tire running under loaded conditions with a quantitatively silica reinforced rubber tread cap, in a tread of cap/base construction, would experience less temperature buildup than a truck tire with a comparable carbon black reinforced rubber tread cap.
Inclusion of a small amount of 3,4-polyisoprene for such evaluation is to evaluate promoting traction for the rubber composition. Use of 3,4-polyisoprene in rubber composition for tire application purposes may be found, for example, in U.S. Pat. Nos. 5,087,668 and 5,336,730.
Use of a small amount of a salt of at least one fatty acid soap (e.g. product of at least one of stearic, palmitic and oleic acid soap with at least one of zinc oxide, magnesium oxide, magnesium carbonate and calcium carbonate, usually in situ within the rubber composition) for such evaluation is to evaluate promoting easier processing for the uncured rubber composition.
A purpose of providing a designed sulfur cure package for such evaluation is to promote resistance to cure reversion for the rubber composition and therefore a more cure reversion resistant rubber composition.
Therefore, it is desired to innovate and provide an evaluation beyond simply providing precipitated silica reinforcement for a truck tire tread.
The term “phr” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight or rubber, or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer”, where used herein unless otherwise prescribed, are used interchangeably. The terms “rubber composition”, “compounded rubber” and “rubber compound” where used herein unless otherwise prescribed, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients or materials” and such terms are well known to those having skill in the rubber mixing, or rubber compounding, art.
The Tg of a polymer, particularly an elastomer, as used herein unless otherwise prescribed, refers to its glass transition temperature which can conventionally be determined, for example, by a differential scanning calorimeter at heating rate of 15° C. to an observed transition of the temperature versus time curve. It is understood that such Tg determination is well known to those having skill in such art.